Provide a way to disable inclusion of code that relies on private APIs (#283)

Applications that rely on private APIs may and will get rejected by Apple.
This change introduces a feature 'no-private-apis' that, when turned on,
ensures that no private APIs are used. Currently, the only private API
in the codebase seems to be AudioDeviceDuck, but in the future there may
be more.

Caveat: when 'no-private-apis' is turned on, calls to disable audio device
ducking will be removed, but no alternative method to disable ducking is
provided.

Author: emir-signal

Cargo.toml

@@ -29,6 +29,7 @@ audio-dump = []
 vpio-forcelist = []
 gecko-in-tree = ["cubeb-backend/gecko-in-tree"]
 tsan-annotations = []
+no-private-apis = ["coreaudio-sys-utils/no-private-apis"]
 
 # Workaround for https://github.com/rust-lang/cargo/issues/6745 to allow this
 # Cargo.toml file to appear under a subdirectory of a workspace without being in

coreaudio-sys-utils/Cargo.toml

@@ -12,3 +12,6 @@ core-foundation-sys = { version = "0.8" }
 default-features = false
 features = ["audio_unit", "core_audio", "io_kit_audio"]
 version = "0.2.18"
+
+[features]
+no-private-apis = []

coreaudio-sys-utils/src/audio_device_extensions.rs

@@ -1,9 +1,12 @@
+#[allow(unused_imports)]
 use crate::dispatch::*;
+#[allow(unused_imports)]
 use coreaudio_sys::*;
 
 // See https://opensource.apple.com/source/WebCore/WebCore-7604.5.6/platform/spi/cf/CoreAudioSPI.h.auto.html
 // Per https://github.com/WebKit/WebKit/commit/7c4c851bc80f14b4cf907f76d65baee013a45eea,
 // this first appeared in MacOS 10.13 and iOS 11.0.
+#[cfg(not(feature = "no-private-apis"))]
 extern "C" {
     fn AudioDeviceDuck(
         inDevice: AudioDeviceID,
@@ -14,6 +17,7 @@ extern "C" {
 }
 
 // See https://github.com/mozilla/cubeb-coreaudio-rs/issues/237
+#[cfg(not(feature = "no-private-apis"))]
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 pub fn audio_device_duck(
     in_device: AudioDeviceID,

src/backend/mod.rs

@@ -21,6 +21,7 @@ use self::aggregate_device::*;
 use self::auto_release::*;
 use self::buffer_manager::*;
 use self::coreaudio_sys_utils::aggregate_device::*;
+#[allow(unused_imports)]
 use self::coreaudio_sys_utils::audio_device_extensions::*;
 use self::coreaudio_sys_utils::audio_object::*;
 use self::coreaudio_sys_utils::audio_unit::*;
@@ -1515,6 +1516,7 @@ fn create_voiceprocessing_audiounit() -> Result<VoiceProcessingUnit> {
         return Err(Error::Error);
     }
 
+    #[cfg(not(feature = "no-private-apis"))]
     match get_default_device(DeviceType::OUTPUT) {
         None => {
             cubeb_log!("Could not get default output device in order to undo vpio ducking");
@@ -4423,6 +4425,7 @@ impl<'ctx> CoreStreamData<'ctx> {
             }
         }
 
+        #[cfg(not(feature = "no-private-apis"))]
         if using_voice_processing_unit {
             // The VPIO AudioUnit automatically ducks other audio streams on the VPIO
             // output device. Its ramp duration is 0.5s when ducking, so unduck similarly